merge GHC HEAD

[ghc-hetmet.git] / compiler / deSugar / DsExpr.lhs

diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index 043f54f..2ac19ce 100644 (file)
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -26,37 +26,43 @@ import DsUtils
 import DsArrows
 import DsMonad
 import Name
 import DsArrows
 import DsMonad
 import Name

+import NameEnv

 
 #ifdef GHCI
 
 #ifdef GHCI

-import PrelNames

        -- Template Haskell stuff iff bootstrapped
 import DsMeta
 #endif
 
 import HsSyn
        -- Template Haskell stuff iff bootstrapped
 import DsMeta
 #endif
 
 import HsSyn

-import TcHsSyn

 
 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
 --     needs to see source types
 import TcType
 import Type
 
 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
 --     needs to see source types
 import TcType
 import Type

+import Coercion

 import CoreSyn
 import CoreUtils
 import CoreSyn
 import CoreUtils

+import CoreFVs

 import MkCore
 
 import DynFlags
 import MkCore
 
 import DynFlags

+import StaticFlags

 import CostCentre
 import Id
 import CostCentre
 import Id

-import PrelInfo
+import VarSet
+import VarEnv

 import DataCon
 import TysWiredIn
 import BasicTypes
 import PrelNames
 import DataCon
 import TysWiredIn
 import BasicTypes
 import PrelNames

+import Maybes

 import SrcLoc
 import Util
 import Bag
 import Outputable
 import FastString
 import SrcLoc
 import Util
 import Bag
 import Outputable
 import FastString

+
+import Control.Monad

 \end{code}
 
 
 \end{code}
 
 

@@ -78,9 +84,9 @@ dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
 
 -------------------------
 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
 
 -------------------------
 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr

-dsIPBinds (IPBinds ip_binds dict_binds) body
-  = do { prs <- dsLHsBinds dict_binds
-       ; let inner = Let (Rec prs) body
+dsIPBinds (IPBinds ip_binds ev_binds) body
+  = do { ds_ev_binds <- dsTcEvBinds ev_binds
+       ; let inner = wrapDsEvBinds ds_ev_binds body

                -- The dict bindings may not be in 
                -- dependency order; hence Rec
        ; foldrM ds_ip_bind inner ip_binds }
                -- The dict bindings may not be in 
                -- dependency order; hence Rec
        ; foldrM ds_ip_bind inner ip_binds }

@@ -96,50 +102,18 @@ ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
 -- a tuple and doing selections.
 -- Silently ignore INLINE and SPECIALISE pragmas...
 ds_val_bind (NonRecursive, hsbinds) body
 -- a tuple and doing selections.
 -- Silently ignore INLINE and SPECIALISE pragmas...
 ds_val_bind (NonRecursive, hsbinds) body

-  | [L _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
-    (L loc bind : null_binds) <- bagToList binds,
-    isBangHsBind bind
-    || isUnboxedTupleBind bind
-    || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
-  = let
-      body_w_exports                 = foldr bind_export body exports
-      bind_export (tvs, g, l, _) body = ASSERT( null tvs )
-                                       bindNonRec g (Var l) body
-    in
-    ASSERT (null null_binds)
+  | [L loc bind] <- bagToList hsbinds,

        -- Non-recursive, non-overloaded bindings only come in ones
        -- ToDo: in some bizarre case it's conceivable that there
        --       could be dict binds in the 'binds'.  (See the notes
        --       below.  Then pattern-match would fail.  Urk.)
        -- Non-recursive, non-overloaded bindings only come in ones
        -- ToDo: in some bizarre case it's conceivable that there
        --       could be dict binds in the 'binds'.  (See the notes
        --       below.  Then pattern-match would fail.  Urk.)

-    putSrcSpanDs loc   $
-    case bind of
-      FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn, 
-               fun_tick = tick, fun_infix = inf }
-        -> do (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
-              MASSERT( null args ) -- Functions aren't lifted
-              MASSERT( isIdHsWrapper co_fn )
-              rhs' <- mkOptTickBox tick rhs
-              return (bindNonRec fun rhs' body_w_exports)
-
-      PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }
-       ->      -- let C x# y# = rhs in body
-               -- ==> case rhs of C x# y# -> body
-          putSrcSpanDs loc                     $
-           do { rhs <- dsGuarded grhss ty
-              ; let upat = unLoc pat
-                    eqn = EqnInfo { eqn_pats = [upat], 
-                                    eqn_rhs = cantFailMatchResult body_w_exports }
-              ; var    <- selectMatchVar upat
-              ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
-              ; return (scrungleMatch var rhs result) }
-
-      _ -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
-
+    strictMatchOnly bind
+  = putSrcSpanDs loc (dsStrictBind bind body)

 
 -- Ordinary case for bindings; none should be unlifted
 ds_val_bind (_is_rec, binds) body
   = do { prs <- dsLHsBinds binds
 
 -- Ordinary case for bindings; none should be unlifted
 ds_val_bind (_is_rec, binds) body
   = do { prs <- dsLHsBinds binds

-       ; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
+       ; ASSERT2( not (any (isUnLiftedType . idType . fst) prs), ppr _is_rec $$ ppr binds )

          case prs of
             [] -> return body
             _  -> return (Let (Rec prs) body) }
          case prs of
             [] -> return body
             _  -> return (Let (Rec prs) body) }

@@ -154,9 +128,53 @@ ds_val_bind (_is_rec, binds) body
        -- NB The previous case dealt with unlifted bindings, so we
        --    only have to deal with lifted ones now; so Rec is ok
 
        -- NB The previous case dealt with unlifted bindings, so we
        --    only have to deal with lifted ones now; so Rec is ok
 

-isUnboxedTupleBind :: HsBind Id -> Bool
-isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
-isUnboxedTupleBind _                             = False
+------------------
+dsStrictBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
+dsStrictBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
+               , abs_exports = exports
+               , abs_ev_binds = ev_binds
+               , abs_binds = binds }) body
+  = do { ds_ev_binds <- dsTcEvBinds ev_binds
+       ; let body1 = foldr bind_export body exports
+             bind_export (_, g, l, _) b = bindNonRec g (Var l) b
+       ; body2 <- foldlBagM (\body bind -> dsStrictBind (unLoc bind) body) 
+                            body1 binds 
+       ; return (wrapDsEvBinds ds_ev_binds body2) }
+
+dsStrictBind (FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn 
+                     , fun_tick = tick, fun_infix = inf }) body
+               -- Can't be a bang pattern (that looks like a PatBind)
+               -- so must be simply unboxed
+  = do { (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
+       ; MASSERT( null args ) -- Functions aren't lifted
+       ; MASSERT( isIdHsWrapper co_fn )
+       ; rhs' <- mkOptTickBox tick rhs
+       ; return (bindNonRec fun rhs' body) }
+
+dsStrictBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
+  =    -- let C x# y# = rhs in body
+       -- ==> case rhs of C x# y# -> body
+    do { rhs <- dsGuarded grhss ty
+       ; let upat = unLoc pat
+             eqn = EqnInfo { eqn_pats = [upat], 
+                             eqn_rhs = cantFailMatchResult body }
+       ; var    <- selectMatchVar upat
+       ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
+       ; return (scrungleMatch var rhs result) }
+
+dsStrictBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
+
+----------------------
+strictMatchOnly :: HsBind Id -> Bool
+strictMatchOnly (AbsBinds { abs_binds = binds })
+  = anyBag (strictMatchOnly . unLoc) binds
+strictMatchOnly (PatBind { pat_lhs = lpat, pat_rhs_ty = ty })
+  =  isUnboxedTupleType ty 
+  || isBangLPat lpat   
+  || any (isUnLiftedType . idType) (collectPatBinders lpat)
+strictMatchOnly (FunBind { fun_id = L _ id })
+  = isUnLiftedType (idType id)
+strictMatchOnly _ = False -- I hope!  Checked immediately by caller in fact

 
 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
 -- Returns something like (let var = scrut in body)
 
 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
 -- Returns something like (let var = scrut in body)

@@ -198,12 +216,28 @@ dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
 
 dsExpr :: HsExpr Id -> DsM CoreExpr
 dsExpr (HsPar e)             = dsLExpr e
 
 dsExpr :: HsExpr Id -> DsM CoreExpr
 dsExpr (HsPar e)             = dsLExpr e

+
+dsExpr (HsHetMetBrak c   e)   = do { e' <- dsExpr (unLoc e)
+                                 ; brak <- dsLookupGlobalId hetmet_brak_name
+                                 ; return $ mkApps (Var brak) [ (Type c), (Type $ exprType e'), e'] }
+dsExpr (HsHetMetEsc  c t e)   = do { e' <- dsExpr (unLoc e)
+                                 ; esc <- dsLookupGlobalId hetmet_esc_name
+                                 ; return $ mkApps (Var esc)  [ (Type c), (Type t), e'] }
+dsExpr (HsHetMetCSP  c   e)   = do { e' <- dsExpr (unLoc e)
+                                 ; csp <- dsLookupGlobalId hetmet_csp_name
+                                 ; return $ mkApps (Var csp)  [ (Type c), (Type $ exprType e'), e'] }

 dsExpr (ExprWithTySigOut e _) = dsLExpr e
 dsExpr (HsVar var)                   = return (Var var)
 dsExpr (HsIPVar ip)                  = return (Var (ipNameName ip))
 dsExpr (HsLit lit)                   = dsLit lit
 dsExpr (HsOverLit lit)               = dsOverLit lit
 dsExpr (ExprWithTySigOut e _) = dsLExpr e
 dsExpr (HsVar var)                   = return (Var var)
 dsExpr (HsIPVar ip)                  = return (Var (ipNameName ip))
 dsExpr (HsLit lit)                   = dsLit lit
 dsExpr (HsOverLit lit)               = dsOverLit lit

-dsExpr (HsWrap co_fn e)       = dsCoercion co_fn (dsExpr e)
+
+dsExpr (HsWrap co_fn e)
+  = do { co_fn' <- dsHsWrapper co_fn
+       ; e' <- dsExpr e
+       ; warn_id <- doptDs Opt_WarnIdentities
+       ; when warn_id $ warnAboutIdentities e' co_fn'
+       ; return (co_fn' e') }

 
 dsExpr (NegApp expr neg_expr) 
   = App <$> dsExpr neg_expr <*> dsLExpr expr
 
 dsExpr (NegApp expr neg_expr) 
   = App <$> dsExpr neg_expr <*> dsLExpr expr

@@ -212,7 +246,7 @@ dsExpr (HsLam a_Match)
   = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
 
 dsExpr (HsApp fun arg)
   = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
 
 dsExpr (HsApp fun arg)

-  = mkCoreApp <$> dsLExpr fun <*>  dsLExpr arg
+  = mkCoreAppDs <$> dsLExpr fun <*>  dsLExpr arg

 \end{code}
 
 Operator sections.  At first it looks as if we can convert
 \end{code}
 
 Operator sections.  At first it looks as if we can convert

@@ -239,10 +273,10 @@ will sort it out.
 \begin{code}
 dsExpr (OpApp e1 op _ e2)
   = -- for the type of y, we need the type of op's 2nd argument
 \begin{code}
 dsExpr (OpApp e1 op _ e2)
   = -- for the type of y, we need the type of op's 2nd argument

-    mkCoreApps <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
+    mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]

     
 dsExpr (SectionL expr op)      -- Desugar (e !) to ((!) e)
     
 dsExpr (SectionL expr op)      -- Desugar (e !) to ((!) e)

-  = mkCoreApp <$> dsLExpr op <*> dsLExpr expr
+  = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr

 
 -- dsLExpr (SectionR op expr)  -- \ x -> op x expr
 dsExpr (SectionR op expr) = do
 
 -- dsLExpr (SectionR op expr)  -- \ x -> op x expr
 dsExpr (SectionR op expr) = do

@@ -254,22 +288,43 @@ dsExpr (SectionR op expr) = do
     x_id <- newSysLocalDs x_ty
     y_id <- newSysLocalDs y_ty
     return (bindNonRec y_id y_core $
     x_id <- newSysLocalDs x_ty
     y_id <- newSysLocalDs y_ty
     return (bindNonRec y_id y_core $

-            Lam x_id (mkCoreApps core_op [Var x_id, Var y_id]))
+            Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
+
+dsExpr (ExplicitTuple tup_args boxity)
+  = do { let go (lam_vars, args) (Missing ty)
+                    -- For every missing expression, we need
+                   -- another lambda in the desugaring.
+               = do { lam_var <- newSysLocalDs ty
+                    ; return (lam_var : lam_vars, Var lam_var : args) }
+            go (lam_vars, args) (Present expr)
+                   -- Expressions that are present don't generate
+                    -- lambdas, just arguments.
+               = do { core_expr <- dsLExpr expr
+                    ; return (lam_vars, core_expr : args) }
+
+       ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
+               -- The reverse is because foldM goes left-to-right
+
+       ; return $ mkCoreLams lam_vars $ 
+                  mkConApp (tupleCon boxity (length tup_args))
+                           (map (Type . exprType) args ++ args) }

 
 dsExpr (HsSCC cc expr) = do
     mod_name <- getModuleDs
     Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
 
 
 dsExpr (HsSCC cc expr) = do
     mod_name <- getModuleDs
     Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
 

-
--- hdaume: core annotation
-

 dsExpr (HsCoreAnn fs expr)
   = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
 
 dsExpr (HsCoreAnn fs expr)
   = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
 

-dsExpr (HsCase discrim matches) = do
-    core_discrim <- dsLExpr discrim
-    ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
-    return (scrungleMatch discrim_var core_discrim matching_code)
+dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty)) 
+  | isEmptyMatchGroup matches  -- A Core 'case' is always non-empty
+  =                            -- So desugar empty HsCase to error call
+    mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
+
+  | otherwise
+  = do { core_discrim <- dsLExpr discrim
+       ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
+       ; return (scrungleMatch discrim_var core_discrim matching_code) }

 
 -- Pepe: The binds are in scope in the body but NOT in the binding group
 --       This is to avoid silliness in breakpoints
 
 -- Pepe: The binds are in scope in the body but NOT in the binding group
 --       This is to avoid silliness in breakpoints

@@ -280,26 +335,21 @@ dsExpr (HsLet binds body) = do
 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
 -- because the interpretation of `stmts' depends on what sort of thing it is.
 --
 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
 -- because the interpretation of `stmts' depends on what sort of thing it is.
 --

-dsExpr (HsDo ListComp stmts body result_ty)
-  =    -- Special case for list comprehensions
-    dsListComp stmts body elt_ty
-  where
-    [elt_ty] = tcTyConAppArgs result_ty
-
-dsExpr (HsDo DoExpr stmts body result_ty)
-  = dsDo stmts body result_ty
-
-dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
-  = dsMDo tbl stmts body result_ty
-
-dsExpr (HsDo PArrComp stmts body result_ty)
-  =    -- Special case for array comprehensions
-    dsPArrComp (map unLoc stmts) body elt_ty
-  where
-    [elt_ty] = tcTyConAppArgs result_ty
-
-dsExpr (HsIf guard_expr then_expr else_expr)
-  = mkIfThenElse <$> dsLExpr guard_expr <*> dsLExpr then_expr <*> dsLExpr else_expr
+dsExpr (HsDo ListComp  stmts res_ty) = dsListComp stmts res_ty
+dsExpr (HsDo PArrComp  stmts _)      = dsPArrComp (map unLoc stmts)
+dsExpr (HsDo DoExpr    stmts _)      = dsDo stmts 
+dsExpr (HsDo GhciStmt  stmts _)      = dsDo stmts 
+dsExpr (HsDo MDoExpr   stmts _)      = dsDo stmts 
+dsExpr (HsDo MonadComp stmts _)      = dsMonadComp stmts
+
+dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
+  = do { pred <- dsLExpr guard_expr
+       ; b1 <- dsLExpr then_expr
+       ; b2 <- dsLExpr else_expr
+       ; case mb_fun of
+           Just fun -> do { core_fun <- dsExpr fun
+                          ; return (mkCoreApps core_fun [pred,b1,b2]) }
+           Nothing  -> return $ mkIfThenElse pred b1 b2 }

 \end{code}
 
 
 \end{code}
 
 

@@ -310,25 +360,20 @@ dsExpr (HsIf guard_expr then_expr else_expr)
 dsExpr (ExplicitList elt_ty xs) 
   = dsExplicitList elt_ty xs
 
 dsExpr (ExplicitList elt_ty xs) 
   = dsExplicitList elt_ty xs
 

--- we create a list from the array elements and convert them into a list using
--- `PrelPArr.toP'
---
---  * the main disadvantage to this scheme is that `toP' traverses the list
---   twice: once to determine the length and a second time to put to elements
---   into the array; this inefficiency could be avoided by exposing some of
---   the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
---   that we can exploit the fact that we already know the length of the array
---   here at compile time
+-- We desugar [:x1, ..., xn:] as
+--   singletonP x1 +:+ ... +:+ singletonP xn

 --
 --

+dsExpr (ExplicitPArr ty []) = do
+    emptyP <- dsLookupDPHId emptyPName
+    return (Var emptyP `App` Type ty)

 dsExpr (ExplicitPArr ty xs) = do
 dsExpr (ExplicitPArr ty xs) = do

-    toP <- dsLookupGlobalId toPName
-    coreList <- dsExpr (ExplicitList ty xs)
-    return (mkApps (Var toP) [Type ty, coreList])
-
-dsExpr (ExplicitTuple expr_list boxity) = do
-    core_exprs <- mapM dsLExpr expr_list
-    return (mkConApp (tupleCon boxity (length expr_list))
-                  (map (Type .  exprType) core_exprs ++ core_exprs))
+    singletonP <- dsLookupDPHId singletonPName
+    appP       <- dsLookupDPHId appPName
+    xs'        <- mapM dsLExpr xs
+    return . foldr1 (binary appP) $ map (unary singletonP) xs'
+  where
+    unary  fn x   = mkApps (Var fn) [Type ty, x]
+    binary fn x y = mkApps (Var fn) [Type ty, x, y]

 
 dsExpr (ArithSeq expr (From from))
   = App <$> dsExpr expr <*> dsLExpr from
 
 dsExpr (ArithSeq expr (From from))
   = App <$> dsExpr expr <*> dsLExpr from

@@ -387,8 +432,8 @@ dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
           = case findField (rec_flds rbinds) lbl of
               (rhs:rhss) -> ASSERT( null rhss )
                             dsLExpr rhs
           = case findField (rec_flds rbinds) lbl of
               (rhs:rhss) -> ASSERT( null rhss )
                             dsLExpr rhs

-              []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
-        unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
+              []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
+        unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty

 
         labels = dataConFieldLabels (idDataCon data_con_id)
         -- The data_con_id is guaranteed to be the wrapper id of the constructor
 
         labels = dataConFieldLabels (idDataCon data_con_id)
         -- The data_con_id is guaranteed to be the wrapper id of the constructor

@@ -421,52 +466,101 @@ RHSs, and do not generate a Core constructor application directly, because the c
 might do some argument-evaluation first; and may have to throw away some
 dictionaries.
 
 might do some argument-evaluation first; and may have to throw away some
 dictionaries.
 

+Note [Update for GADTs]
+~~~~~~~~~~~~~~~~~~~~~~~
+Consider 
+   data T a b where
+     T1 { f1 :: a } :: T a Int
+
+Then the wrapper function for T1 has type 
+   $WT1 :: a -> T a Int
+But if x::T a b, then
+   x { f1 = v } :: T a b   (not T a Int!)
+So we need to cast (T a Int) to (T a b).  Sigh.
+

 \begin{code}
 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
                       cons_to_upd in_inst_tys out_inst_tys)
   | null fields
   = dsLExpr record_expr
   | otherwise
 \begin{code}
 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
                       cons_to_upd in_inst_tys out_inst_tys)
   | null fields
   = dsLExpr record_expr
   | otherwise

-  =    -- Record stuff doesn't work for existentials
-       -- The type checker checks for this, but we need 
-       -- worry only about the constructors that are to be updated
-    ASSERT2( notNull cons_to_upd && all isVanillaDataCon cons_to_upd, ppr expr )
+  = ASSERT2( notNull cons_to_upd, ppr expr )

 
     do { record_expr' <- dsLExpr record_expr
 
     do { record_expr' <- dsLExpr record_expr

-       ; let   -- Awkwardly, for families, the match goes 
-               -- from instance type to family type
-               tycon     = dataConTyCon (head cons_to_upd)
-               in_ty     = mkTyConApp tycon in_inst_tys
-               in_out_ty = mkFunTy in_ty
-                                   (mkFamilyTyConApp tycon out_inst_tys)
-
-               mk_val_arg field old_arg_id 
-                 = case findField fields field  of
-                     (rhs:rest) -> ASSERT(null rest) rhs
-                     []         -> nlHsVar old_arg_id
-
-               mk_alt con
-                 = ASSERT( isVanillaDataCon con )
-                   do  { arg_ids <- newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys)
-                       -- This call to dataConInstOrigArgTys won't work for existentials
-                       -- but existentials don't have record types anyway
-                       ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
-                                               (dataConFieldLabels con) arg_ids
-                             rhs = foldl (\a b -> nlHsApp a b)
-                                         (nlHsTyApp (dataConWrapId con) out_inst_tys)
-                                         val_args
-                             pat = mkPrefixConPat con (map nlVarPat arg_ids) in_ty
-
-                       ; return (mkSimpleMatch [pat] rhs) }
+       ; field_binds' <- mapM ds_field fields
+       ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
+             upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']

 
        -- It's important to generate the match with matchWrapper,
        -- and the right hand sides with applications of the wrapper Id
        -- so that everything works when we are doing fancy unboxing on the
        -- constructor aguments.
 
        -- It's important to generate the match with matchWrapper,
        -- and the right hand sides with applications of the wrapper Id
        -- so that everything works when we are doing fancy unboxing on the
        -- constructor aguments.

-       ; alts <- mapM mk_alt cons_to_upd
-       ; ([discrim_var], matching_code) <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
+       ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
+       ; ([discrim_var], matching_code) 
+               <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
+
+       ; return (add_field_binds field_binds' $
+                 bindNonRec discrim_var record_expr' matching_code) }
+  where
+    ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
+      -- Clone the Id in the HsRecField, because its Name is that
+      -- of the record selector, and we must not make that a lcoal binder
+      -- else we shadow other uses of the record selector
+      -- Hence 'lcl_id'.  Cf Trac #2735
+    ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
+                           ; let fld_id = unLoc (hsRecFieldId rec_field)
+                           ; lcl_id <- newSysLocalDs (idType fld_id)
+                           ; return (idName fld_id, lcl_id, rhs) }
+
+    add_field_binds [] expr = expr
+    add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
+
+       -- Awkwardly, for families, the match goes 
+       -- from instance type to family type
+    tycon     = dataConTyCon (head cons_to_upd)
+    in_ty     = mkTyConApp tycon in_inst_tys
+    in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
+
+    mk_alt upd_fld_env con
+      = do { let (univ_tvs, ex_tvs, eq_spec, 
+                 theta, arg_tys, _) = dataConFullSig con
+                subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
+
+               -- I'm not bothering to clone the ex_tvs
+          ; eqs_vars   <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
+          ; theta_vars <- mapM newPredVarDs (substTheta subst theta)
+          ; arg_ids    <- newSysLocalsDs (substTys subst arg_tys)
+          ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
+                                        (dataConFieldLabels con) arg_ids
+                 mk_val_arg field_name pat_arg_id 
+                     = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
+                inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
+                       -- Reconstruct with the WrapId so that unpacking happens
+                wrap = mkWpEvVarApps theta_vars          `WpCompose` 
+                       mkWpTyApps    (mkTyVarTys ex_tvs) `WpCompose`
+                       mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
+                                      , not (tv `elemVarEnv` wrap_subst) ]
+                rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
+
+                       -- Tediously wrap the application in a cast
+                       -- Note [Update for GADTs]
+                wrapped_rhs | null eq_spec = rhs
+                            | otherwise    = mkLHsWrap (WpCast wrap_co) rhs
+                wrap_co = mkTyConAppCo tycon [ lookup tv ty
+                                             | (tv,ty) <- univ_tvs `zip` out_inst_tys]
+                lookup univ_tv ty = case lookupVarEnv wrap_subst univ_tv of
+                                       Just co' -> co'
+                                       Nothing  -> mkReflCo ty
+                wrap_subst = mkVarEnv [ (tv, mkSymCo (mkCoVarCo co_var))
+                                      | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
+
+                pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
+                                        , pat_dicts = eqs_vars ++ theta_vars
+                                        , pat_binds = emptyTcEvBinds
+                                        , pat_args = PrefixCon $ map nlVarPat arg_ids
+                                        , pat_ty = in_ty }
+          ; return (mkSimpleMatch [pat] wrapped_rhs) }

 
 

-       ; return (bindNonRec discrim_var record_expr' matching_code) }

 \end{code}
 
 Here is where we desugar the Template Haskell brackets and escapes
 \end{code}
 
 Here is where we desugar the Template Haskell brackets and escapes

@@ -499,7 +593,7 @@ dsExpr (HsTick ix vars e) = do
 
 dsExpr (HsBinTick ixT ixF e) = do
   e2 <- dsLExpr e
 
 dsExpr (HsBinTick ixT ixF e) = do
   e2 <- dsLExpr e

-  do { ASSERT(exprType e2 `coreEqType` boolTy)
+  do { ASSERT(exprType e2 `eqType` boolTy)

        mkBinaryTickBox ixT ixF e2
      }
 \end{code}
        mkBinaryTickBox ixT ixF e2
      }
 \end{code}

@@ -552,24 +646,53 @@ allocation in some nofib programs. Specifically
 
 Of course, if rules aren't turned on then there is pretty much no
 point doing this fancy stuff, and it may even be harmful.
 
 Of course, if rules aren't turned on then there is pretty much no
 point doing this fancy stuff, and it may even be harmful.

-\begin{code}

 
 

+=======>  Note by SLPJ Dec 08.
+
+I'm unconvinced that we should *ever* generate a build for an explicit
+list.  See the comments in GHC.Base about the foldr/cons rule, which 
+points out that (foldr k z [a,b,c]) may generate *much* less code than
+(a `k` b `k` c `k` z).
+
+Furthermore generating builds messes up the LHS of RULES. 
+Example: the foldr/single rule in GHC.Base
+   foldr k z [x] = ...
+We do not want to generate a build invocation on the LHS of this RULE!
+
+We fix this by disabling rules in rule LHSs, and testing that
+flag here; see Note [Desugaring RULE left hand sides] in Desugar
+
+To test this I've added a (static) flag -fsimple-list-literals, which
+makes all list literals be generated via the simple route.  
+
+
+\begin{code}

 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
 -- See Note [Desugaring explicit lists]
 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
 -- See Note [Desugaring explicit lists]

-dsExplicitList elt_ty xs = do
-    dflags <- getDOptsDs
-    xs' <- mapM dsLExpr xs
-    if not (dopt Opt_RewriteRules dflags)
-        then return $ mkListExpr elt_ty xs'
-        else mkBuildExpr elt_ty (mkSplitExplicitList (thisPackage dflags) xs')
+dsExplicitList elt_ty xs
+  = do { dflags <- getDOptsDs
+       ; xs' <- mapM dsLExpr xs
+       ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
+       ; if opt_SimpleListLiterals                     -- -fsimple-list-literals
+         || not (dopt Opt_EnableRewriteRules dflags)   -- Rewrite rules off
+               -- Don't generate a build if there are no rules to eliminate it!
+               -- See Note [Desugaring RULE left hand sides] in Desugar
+         || null dynamic_prefix   -- Avoid build (\c n. foldr c n xs)!
+         then return $ mkListExpr elt_ty xs'
+         else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }

   where
   where

-    mkSplitExplicitList this_package xs' (c, _) (n, n_ty) = do
-        let (dynamic_prefix, static_suffix) = spanTail (rhsIsStatic this_package) xs'
-            static_suffix' = mkListExpr elt_ty static_suffix
-        
-        folded_static_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) static_suffix'
-        let build_body = foldr (App . App (Var c)) folded_static_suffix dynamic_prefix
-        return build_body
+    is_static :: CoreExpr -> Bool
+    is_static e = all is_static_var (varSetElems (exprFreeVars e))
+
+    is_static_var :: Var -> Bool
+    is_static_var v 
+      | isId v = isExternalName (idName v)  -- Top-level things are given external names
+      | otherwise = False                   -- Type variables
+
+    mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
+      = do { let suffix' = mkListExpr elt_ty suffix
+           ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
+           ; return (foldr (App . App (Var c)) folded_suffix prefix) }

 
 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
 spanTail f xs = (reverse rejected, reverse satisfying)
 
 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
 spanTail f xs = (reverse rejected, reverse satisfying)

@@ -581,150 +704,150 @@ handled in DsListComp).  Basically does the translation given in the
 Haskell 98 report:
 
 \begin{code}
 Haskell 98 report:
 
 \begin{code}

-dsDo   :: [LStmt Id]
-       -> LHsExpr Id
-       -> Type                 -- Type of the whole expression
-       -> DsM CoreExpr
-
-dsDo stmts body _result_ty
-  = go (map unLoc stmts)
+dsDo :: [LStmt Id] -> DsM CoreExpr
+dsDo stmts
+  = goL stmts

   where
   where

-    go [] = dsLExpr body
-    
-    go (ExprStmt rhs then_expr _ : stmts)
+    goL [] = panic "dsDo"
+    goL (L loc stmt:lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
+  
+    go _ (LastStmt body _) stmts
+      = ASSERT( null stmts ) dsLExpr body
+        -- The 'return' op isn't used for 'do' expressions
+
+    go _ (ExprStmt rhs then_expr _ _) stmts

       = do { rhs2 <- dsLExpr rhs
       = do { rhs2 <- dsLExpr rhs

-          ; then_expr2 <- dsExpr then_expr
-          ; rest <- go stmts
+           ; warnDiscardedDoBindings rhs (exprType rhs2) 
+           ; then_expr2 <- dsExpr then_expr
+          ; rest <- goL stmts

           ; return (mkApps then_expr2 [rhs2, rest]) }
     
           ; return (mkApps then_expr2 [rhs2, rest]) }
     

-    go (LetStmt binds : stmts)
-      = do { rest <- go stmts
+    go _ (LetStmt binds) stmts
+      = do { rest <- goL stmts

           ; dsLocalBinds binds rest }
 
           ; dsLocalBinds binds rest }
 

-    go (BindStmt pat rhs bind_op fail_op : stmts)
-      = 
-       do  { body     <- go stmts
-           ; rhs'     <- dsLExpr rhs
-          ; bind_op' <- dsExpr bind_op
-          ; var   <- selectSimpleMatchVarL pat
-          ; let bind_ty = exprType bind_op'    -- rhs -> (pat -> res1) -> res2
-                res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
-          ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
-                                    res1_ty (cantFailMatchResult body)
-          ; match_code <- handle_failure pat match fail_op
-          ; return (mkApps bind_op' [rhs', Lam var match_code]) }
+    go _ (BindStmt pat rhs bind_op fail_op) stmts
+      = do  { body     <- goL stmts
+            ; rhs'     <- dsLExpr rhs
+           ; bind_op' <- dsExpr bind_op
+           ; var   <- selectSimpleMatchVarL pat
+           ; let bind_ty = exprType bind_op'   -- rhs -> (pat -> res1) -> res2
+                 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
+           ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
+                                     res1_ty (cantFailMatchResult body)
+           ; match_code <- handle_failure pat match fail_op
+           ; return (mkApps bind_op' [rhs', Lam var match_code]) }

     
     

+    go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
+                    , recS_rec_ids = rec_ids, recS_ret_fn = return_op
+                    , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
+                    , recS_rec_rets = rec_rets, recS_ret_ty = body_ty }) stmts
+      = ASSERT( length rec_ids > 0 )
+        goL (new_bind_stmt : stmts)
+      where
+        new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats)
+                                         mfix_app bind_op 
+                                         noSyntaxExpr  -- Tuple cannot fail
+
+        tup_ids      = rec_ids ++ filterOut (`elem` rec_ids) later_ids
+        tup_ty       = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
+        rec_tup_pats = map nlVarPat tup_ids
+        later_pats   = rec_tup_pats
+        rets         = map noLoc rec_rets
+        mfix_app     = nlHsApp (noLoc mfix_op) mfix_arg
+        mfix_arg     = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
+                                                 (mkFunTy tup_ty body_ty))
+        mfix_pat     = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
+        body         = noLoc $ HsDo DoExpr (rec_stmts ++ [ret_stmt]) body_ty
+        ret_app      = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
+        ret_stmt     = noLoc $ mkLastStmt ret_app
+                    -- This LastStmt will be desugared with dsDo, 
+                    -- which ignores the return_op in the LastStmt,
+                    -- so we must apply the return_op explicitly 
+
+handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr

     -- In a do expression, pattern-match failure just calls
     -- the monadic 'fail' rather than throwing an exception
     -- In a do expression, pattern-match failure just calls
     -- the monadic 'fail' rather than throwing an exception

-    handle_failure pat match fail_op
-      | matchCanFail match
-      = do { fail_op' <- dsExpr fail_op
-          ; fail_msg <- mkStringExpr (mk_fail_msg pat)
-          ; extractMatchResult match (App fail_op' fail_msg) }
-      | otherwise
-      = extractMatchResult match (error "It can't fail") 
+handle_failure pat match fail_op
+  | matchCanFail match
+  = do { fail_op' <- dsExpr fail_op
+       ; fail_msg <- mkStringExpr (mk_fail_msg pat)
+       ; extractMatchResult match (App fail_op' fail_msg) }
+  | otherwise
+  = extractMatchResult match (error "It can't fail")

 
 mk_fail_msg :: Located e -> String
 mk_fail_msg pat = "Pattern match failure in do expression at " ++ 
                  showSDoc (ppr (getLoc pat))
 \end{code}
 
 
 mk_fail_msg :: Located e -> String
 mk_fail_msg pat = "Pattern match failure in do expression at " ++ 
                  showSDoc (ppr (getLoc pat))
 \end{code}
 

-Translation for RecStmt's: 
------------------------------
-We turn (RecStmt [v1,..vn] stmts) into:
-  
-  (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
-                                     return (v1,..vn))

 
 

+%************************************************************************
+%*                                                                     *
+                 Warning about identities
+%*                                                                     *
+%************************************************************************
+
+Warn about functions that convert between one type and another
+when the to- and from- types are the same.  Then it's probably
+(albeit not definitely) the identity

 \begin{code}
 \begin{code}

-dsMDo  :: PostTcTable
-       -> [LStmt Id]
-       -> LHsExpr Id
-       -> Type                 -- Type of the whole expression
-       -> DsM CoreExpr
-
-dsMDo tbl stmts body result_ty
-  = go (map unLoc stmts)
-  where
-    (m_ty, b_ty) = tcSplitAppTy result_ty      -- result_ty must be of the form (m b)
-    mfix_id   = lookupEvidence tbl mfixName
-    return_id = lookupEvidence tbl returnMName
-    bind_id   = lookupEvidence tbl bindMName
-    then_id   = lookupEvidence tbl thenMName
-    fail_id   = lookupEvidence tbl failMName
-    ctxt      = MDoExpr tbl
-
-    go [] = dsLExpr body
-    
-    go (LetStmt binds : stmts)
-      = do { rest <- go stmts
-          ; dsLocalBinds binds rest }
+warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
+warnAboutIdentities (Var v) co_fn
+  | idName v `elem` conversionNames
+  , let fun_ty = exprType (co_fn (Var v))
+  , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
+  , arg_ty `eqType` res_ty  -- So we are converting  ty -> ty
+  = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
+                 , nest 2 $ ptext (sLit "can probably be omitted")
+                 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
+           ])
+warnAboutIdentities _ _ = return ()
+
+conversionNames :: [Name]
+conversionNames
+  = [ toIntegerName, toRationalName
+    , fromIntegralName, realToFracName ]
+ -- We can't easily add fromIntegerName, fromRationalName,
+ -- becuase they are generated by literals
+\end{code}

 
 

-    go (ExprStmt rhs _ rhs_ty : stmts)
-      = do { rhs2 <- dsLExpr rhs
-          ; rest <- go stmts
-          ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
-    
-    go (BindStmt pat rhs _ _ : stmts)
-      = do { body  <- go stmts
-          ; var   <- selectSimpleMatchVarL pat
-          ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
-                                 result_ty (cantFailMatchResult body)
-          ; fail_msg   <- mkStringExpr (mk_fail_msg pat)
-          ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
-          ; match_code <- extractMatchResult match fail_expr
-
-          ; rhs'       <- dsLExpr rhs
-          ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty, 
-                                            rhs', Lam var match_code]) }
-    
-    go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
-      = ASSERT( length rec_ids > 0 )
-        ASSERT( length rec_ids == length rec_rets )
-       go (new_bind_stmt : let_stmt : stmts)
-      where
-        new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
-       let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
-
-       
-               -- Remove the later_ids that appear (without fancy coercions) 
-               -- in rec_rets, because there's no need to knot-tie them separately
-               -- See Note [RecStmt] in HsExpr
-       later_ids'   = filter (`notElem` mono_rec_ids) later_ids
-       mono_rec_ids = [ id | HsVar id <- rec_rets ]
-    
-       mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
-       mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
-                                            (mkFunTy tup_ty body_ty))
-
-       -- The rec_tup_pat must bind the rec_ids only; remember that the 
-       --      trimmed_laters may share the same Names
-       -- Meanwhile, the later_pats must bind the later_vars
-       rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
-       later_pats   = map nlVarPat    later_ids' ++ map mk_later_pat rec_ids
-       rets         = map nlHsVar     later_ids' ++ map noLoc rec_rets
-
-       mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
-       body     = noLoc $ HsDo ctxt rec_stmts return_app body_ty
-       body_ty = mkAppTy m_ty tup_ty
-       tup_ty  = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
-                 -- mkCoreTupTy deals with singleton case
-
-       return_app  = nlHsApp (nlHsTyApp return_id [tup_ty]) 
-                             (mk_ret_tup rets)
-
-       mk_wild_pat :: Id -> LPat Id 
-       mk_wild_pat v = noLoc $ WildPat $ idType v
-
-       mk_later_pat :: Id -> LPat Id
-       mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
-                      | otherwise           = nlVarPat v
-
-       mk_tup_pat :: [LPat Id] -> LPat Id
-       mk_tup_pat [p] = p
-       mk_tup_pat ps  = noLoc $ mkVanillaTuplePat ps Boxed
-
-       mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
-       mk_ret_tup [r] = r
-       mk_ret_tup rs  = noLoc $ ExplicitTuple rs Boxed
+%************************************************************************
+%*                                                                     *
+\subsection{Errors and contexts}
+%*                                                                     *
+%************************************************************************
+
+\begin{code}
+-- Warn about certain types of values discarded in monadic bindings (#3263)
+warnDiscardedDoBindings :: LHsExpr Id -> Type -> DsM ()
+warnDiscardedDoBindings rhs rhs_ty
+  | Just (m_ty, elt_ty) <- tcSplitAppTy_maybe rhs_ty
+  = do {  -- Warn about discarding non-() things in 'monadic' binding
+       ; warn_unused <- doptDs Opt_WarnUnusedDoBind
+       ; if warn_unused && not (isUnitTy elt_ty)
+         then warnDs (unusedMonadBind rhs elt_ty)
+         else 
+         -- Warn about discarding m a things in 'monadic' binding of the same type,
+         -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
+    do { warn_wrong <- doptDs Opt_WarnWrongDoBind
+       ; case tcSplitAppTy_maybe elt_ty of
+           Just (elt_m_ty, _) | warn_wrong, m_ty `eqType` elt_m_ty
+                              -> warnDs (wrongMonadBind rhs elt_ty)
+           _ -> return () } }
+
+  | otherwise  -- RHS does have type of form (m ty), which is wierd
+  = return ()   -- but at lesat this warning is irrelevant
+
+unusedMonadBind :: LHsExpr Id -> Type -> SDoc
+unusedMonadBind rhs elt_ty
+  = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
+    ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
+    ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
+
+wrongMonadBind :: LHsExpr Id -> Type -> SDoc
+wrongMonadBind rhs elt_ty
+  = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
+    ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
+    ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")

 \end{code}
 \end{code}